Sound reproduction, and particularly sound reproduction of musical arrangements, continues to evolve. Early sound recordings were recorded on a single or mono channel and subsequently were reproduced by an audio signal generator which generated a mono channel signal and drove a single speaker driver with that signal. Although the mono channel presentation reproduced the recorded sounds or music, the sound was very flat and was not "lifelike". That is, the listener of the music would get a different sound sensation during replay of the recorded music as opposed to the sound sensation which would have been obtained by being in the presence of the performer when the music was performed.
With the advent of stereo recording, i.e., two channel recording, the music could be presented with two channel signals. Stereo sound systems are generally focused around a two channel source presentation utilizing two speaker drivers, one for each stereo channel. The stereo channels were designated as a left channel and a right channel. This generally signified the fact that the two speaker drivers would be separated and one would be to the left or right of the other, and vice versa. A two channel recording, when presented through two speaker drivers driven by two separate signals added the depth and width of music which was not achievable with a mono channel system. Therefore, stereo recording creates greater sound ambience and a more lifelike presentation of the music. With a stereo (two channel) system, the image that is created and perceived in the center is based upon the known concept that the perceived sound source created by the two separated speakers, to the extent that they produce the same or similar information, is midway between these sources. Amplitude (volume level) differences have the effect of moving the perceived center image side-to-side, moving the perceived image toward the higher level (volume) speaker. Further, by directing the speaker drivers inwards toward the listener (known as toe-in), their sound presentations will merge together reinforcing the center image. However, utilizing currently available systems, reinforcement or focusing the center image comes at the expense of degrading the spaciousness of the presentation. Maintaining or enlarging the spaciousness of the sound and taking advantage of the independent left and right portion of the sound recording by directing the speakers directly forward or slightly outwards (toe out), comes at the expense of degrading the perceived center image.
To further enhance the reproduction of music, early attempts were made to create a four channel system which involved a left front channel, a right front channel, a left rear channel and a right rear channel. When the four channel, or quad systems were introduced, they were not greatly accepted by the industry.
Recently, efforts have been directed to enhancing the reproduction of two channel recordings and specifically have been directed to how a listener hears and perceives sound from traditional two channel sources. Several of these efforts have focused upon presenting negative signals to cancel out certain sound. For example, a left cancellation signal would be used to cancel sound heard in the right ear while a right cancellation signal is utilized to cancel sound as heard by the listener in the left ear. Such techniques usually utilize a combination of a left minus right (L-R) or a right minus (R-L) signal to achieve the desired results. The primary problem with currently existing L-R and R-L techniques is that the listener, to receive and perceive cancellation signals, needs to be stationary and within a very narrow effective area or "sweet spot". As the listener and the corresponding listening position strays outside of the effective area, the signals lose their ability to cancel because the timing and strengths of the signals are not synchronized.
Other attempts have been made to address the sound levels which a listener hears based upon their position within the room and their proximity to the speaker drivers. These attempts utilize reflected sound, that is, sound bounced off back and side walls. However, such a focus is directed mainly to the levels of the sound and not to the actual integrity of the music. For example, adding additional artificial reflected sound into the room beyond that reflected sound which already exists in the original musical recording, injects a certain artificiality to the replayed music and creates a loss of direction of the source and loss of the musical sound integrity. The characteristics of reflected sound is inherently dependent upon the playback environment and to artificially create additional reflected sound results in signals which are not timed together. Furthermore, the spectrum of sound will differ based upon the reflecting surfaces it encounters when replayed (such as differing wall materials and glass). Therefore, even though existing reflecting sound systems might create more even noise level variations within a room, they do so to the detriment of the musical integrity.
One particular attempt to make sound variations more even as the listener changes positions within a room and with respect to the speaker drivers is set forth in U.S. Pat. No. 5,117,459. The system therein utilizes sound cancellation techniques, creating open "voids" along with left plus right (L+R), and left minus right and right minus left (L-R, R-L) channel signals to achieve its results. While the use of such channel manipulation may possibly result in a more even noise level, the integrity of the music is again sacrificed. The various combinations of left and right signals, including the L-R signal and the R-L signal, along with the L+R signal create an overly vague sound image. Analogizing the mixing of the left and right signal channels with the mixing of colors, the result is a solid or mono sound which does not resemble the original recording similar to the dark or black color which is achieved when various colors of the spectrum are all mixed together. That is, the sound image as replayed is removed from the actual sound images that were recorded. Therefore, past attempts at enhancing a recreated sound image by manipulating the left and right channel signals have been inadequate.
Most recently, three channel recordings and decoding techniques have been utilized for enhanced reproduction of sound and music. These recordings are played by audio systems which produce a left channel signal, a right channel signal and center channel signals. Each of the left, right and center signals has a unique frequency characteristic which is distinct from the other channels. The addition of the center sound channel has further complicated the musical reproduction scheme by requiring a separate speaker driver to play the center channel which must be located with respect to the left and right channel speaker drivers within a room for proper sound reproduction.
One problem with the additional speaker driver for the center channel results from the spatial considerations of placing that speaker driver within a room. Traditionally, it has been sufficient to position one speaker at one side of a room and the other speaker at the other side of a room to create the desired sound image. To operate properly, however, the center speaker must somehow be positioned so that it plays the center channel signal with the right and left channels to produce the desired sound reproduction. As will be appreciated, the location of another speaker driver within a room while still maintaining the aesthetic quality of the room and specific arrangement of furniture, is a real problem. For example, positioning the speaker driver for the center channel in front of or close to a fireplace is not an attractive or practical choice. Furthermore, eliminating the speaker driver for the center channel for these reasons, limits the utilization of the current three channel Art.
Another inherent drawback of reproducing three channel recordings or multiple channel recordings (i.e. recordings with distinct channels that are a combination of the left, right and center channels) with a left speaker driver, a center speaker driver and a right speaker driver is that the center channel often carries a greater amount of information than the left and right channels. Therefore, the center channel information and the respective speaker driver become the predominant piece in the system. As such, this returns the Art (for this center information) back to a mono channel being played from a mono source. The resultant sound image is fixed at a location corresponding to the location of the center channel speaker driver and the overall reproduction tends to be flat and lifeless.
Therefore, it is an objective of the present invention to recreate a musical recording with depth and a more realistic perceived sound image. It is another objective of the present invention to manage the dominance of the center channel in a multi-channel audio system. It is another objective to eliminate the necessity of positioning an additional speaker driver and cabinet within a distinct location separate from the left and right speaker drivers. To that end, it is also an objective to maintain the integrity of the center channel signal and the sound image produced therefrom within the listening area while eliminating the distinct perceived location of a center channel speaker driver. It is another objective of the invention to provide a sound presentation of musical sound images which recreates the sound stage as recorded and reestablishes the depth and the spaciousness of the left to right, right to left, and center sound images to closely resemble live music presentations.